Optical System for Forming an Image in Space

ABSTRACT

An optical system that projects a real image to a location in free space and includes one or more features located along the optical path that enhance the viewability of the real image. The optical system includes a converging element for converging a portion of source light so as to form the real image. One viewability-enhancing features is the use of a broadband reflector-polarizer having high transmitting and reflecting efficiencies. Another viewability-enhancing features is the use of polarizing elements having substantially matched bandwidth responses and/or comprising an achromatic design. An additional viewability-enhancing feature is the use of a wide-view film to increase the viewing angle of the image.

RELATED APPLICATION DATA

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/721,128 filed on Nov. 25, 2003, and titled “Optical SystemFor Forming An Image In Space,” now U.S. Pat. No. 7,242,524 issued Jul.10, 2007, that is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of optics and, moreparticularly, to an optical system for forming an image in space.

BACKGROUND OF THE INVENTION

Optical systems capable of forming images from illuminated objects findnumerous and important applications in areas such as advertising,marketing and product exhibition, as well as other more esotericfunctions such as simulation. It is advantageous for such systems to becompact in size, have a wide field of view and high contrast and beviewable in all ambient lighting conditions.

Many real image optical systems, e.g., real image projectors, aredesigned to create an image wherein the desired image appears against ablack background. In a real image projector for, say, gamingapplications, a floating, real image of a character is projected intospace. The psychological impact of this image is greatest if the gameelements appear bright, sharply defined and of high contrast. An exampleof a current state-of-the-art real image projector of this type is thedual brightness enhancement film (DBEF)-based meniscus-type projectingoptical system disclosed in U.S. Pat. No. 6,262,841 to the presentinventor and shown in FIG. 1 as optical system 20. However, opticalsystem 20, while of very compact design, has a relatively low efficiencyin terms of light throughput and exhibits poor contrast at non-normal,i.e., off-center or oblique, viewing angles.

Generally, the inclusion of DBEF-based optical system 20 herein servesto illustrate the basic function of a real image optical system, as wellas to illuminate the drawbacks of that particular design relative to areal image optical system of the present invention. FIG. 1 shows opticalsystem 20 as including a source light 24 emitted and/or reflected by asource 28. Source light 24 is randomly polarized in nature and issubsequently linearly polarized by a first linear polarizer 32 as ittravels toward a viewer 36. This linearly polarized light passes throughDBEF 40 having its axis aligned in the transmissive orientation withrespect to the linearly polarized light passing through linear polarizer32. The linearly polarized light that passes through DBEF 40 iscircularly polarized by a first quarter wave retarder 44, or firstquarter wave plate (QWP). This circularly polarized light (assumed righthanded for this description) is then made incident upon a partiallymirrored concave (spherical or aspherical) beamsplitter 48. Concavebeamsplitter 48 serves to impart the convergence that ultimately formsthe real image 52.

The reflected portion of the light has its handedness of circularpolarization switched to left handedness by reflection at beamsplitter48 and is converted to linearly polarized light by first quarter waveretarder 44, as it travels right to left in the figure. This linearlypolarized light is largely reflected by DBEF 40, since the direction oflinear polarization is opposite the polarization of the initial linearlypolarized light. This portion of reflected light is then circularlypolarized by first quarter wave retarder 44 as it now travels left toright in the figure. This light is partially reflected and partiallytransmitted by partially-mirrored concave beamsplitter 48. Thetransmitted portion is linearly polarized by a second quarter waveretarder 56. A second linear polarizer 60 is aligned such that thelinearly polarized light is transmitted to form real image 52 apparentto viewer 36.

The portion of the right circularly polarized light transmitted throughpartially-mirrored concave beamsplitter 48 is converted to linearlypolarized light by second quarter wave retarder 56 in a directionopposite to the polarization direction of the finally transmitted light.As the direction of the light that has passed through second quarterwave retarder 56 is opposite the transmissive direction of second linearpolarizer 60, it is extinguished by the second linear polarizer.

Optical system 20 of FIG. 1 utilizes a series of polarizing components,i.e., DBEF 40 and first and second linear polarizers 32, 60 inconjunction with a beamsplitter, i.e., beamsplitter 48, in a compactdesign that redirects light several times to fold the optical path. Thisfolding of the optical path results in a small device size and superiorfield of view. However, DBEF 40 and first and second linear polarizers32, 60 are absorptive in nature and result in losses in terms of lightthroughput. Additionally, the contrast of the final image is affected,especially at oblique viewing angles, by the differing bandwidthresponses among the polarizing elements. The contrast is primarilyaffected by the existence of undesired bleed-through of a portion ofsource light 24 resulting from poor extinction ratios of the polarizingelements in off-angle viewing conditions.

In addition to the shortcomings of optical system 20 just mentioned,there are other performance and manufacturing aspects which can beimproved. For example, the type of quarter wave retarder used for firstand second quarter wave retarders 44, 56 is of a drawn polyvinyl alcohol(PVA) type that characteristically exhibits poor retardance uniformityand has poor performance over time owing to inherent propensity of thesetypes of polarizers to absorb water and thereby alter the retardancevalue. This lack of uniformity results in poor efficiency of the overallsystem that primarily manifests itself, again, as greater bleed-throughat oblique viewing angles.

Optical system 20 also utilizes a flexible first quarter wave retarder44 directly adjacent to DBEF 40, which itself is flexible. Since DBEF 40is used in reflection along the desired light path, it must necessarilybe flat to provide a distortion-free reflected image. However,laminating flexible DBEF 40 directly to flexible first quarter waveretarder 56 results in an undulating DBEF surface and, hence, adistorted reflection. The manufacturing complexity of maintainingflatness in flexible DBEF 40 while laminated (or optically coupled) toflexible first quarter wave retarder 44 on one side and first linearpolarizer 32 on the other is apparent. Thus, DBEF 40 must be laminatedto its own flat glass substrate (not shown) prior to lamination to firstquarter wave retarder 44 and additional polarizing and anti-reflectiveglass elements. This additional step results in optical system 20including a subassembly having three rigid substrates, including twoanti-reflective glass components, along with three sheet-type polarizingelements, resulting in a substantial manufacturing complexity. RegardingDBEF 40 itself, the stand-alone contribution of this element to theoverall throughput of the entire system is about 49%, i.e., about 70%reflection and about 70% transmission.

The portion of the right circularly polarized light that is ideallyextinguished by second linear polarizer 60 is usually not sufficiently,i.e., “cleanly,” polarized to be completely extinguished by thispolarizer. Inefficiencies exist, since current PVA-type polarizers areoptimized at only a single wavelength, whereas source light 24 for theintended applications, e.g., the applications discussed above, istypically polychromatic. Oblique viewing of real image 52 further teststhe limitations of the current quarter wave retarders, since theperformance of this type of retarder is highly viewing-angle dependent.

Accordingly, it is desired to obtain a compact meniscus-type real imageprojector having higher brightness and contrast and bettermanufacturability than optical system 20, while retaining or improvingthe superior system size and field of view characteristics of thatsystem. Several improvements for enhancing the image characteristics ofoptical system of are disclosed herein.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is directed to a method offorming a real image floating in space. The method includes: receivinglight from an image source from a first direction along an optical axis;polarizing the light so as to create circularly polarized light having afirst handedness; bouncing a first portion of the circularly polarizedlight between a broadband reflector-polarizer and a partially mirroredreflector along the optical axis so that the first portion of thecircularly polarized light has a second handedness opposite the firsthandedness and is directed along the optical axis in a second directionopposite the first direction; after the bouncing of the first portion ofthe circularly polarized light, converting the first portion of thecircularly polarized light to linearly polarized light along the opticalaxis; passing a second portion of the circularly polarized light throughthe broadband reflector-polarizer and the partially mirrored reflectoralong the optical axis so that the circularly polarized light retainsthe first handedness; after the passing of the second portion of thecircularly polarized light, extinguishing the second portion of thecircularly polarized light; and focusing the first portion of thecircularly polarized light to a location in space so as to form afloating real image at the location.

In another embodiment, the present invention is directed to a method ofdisplaying a plurality of images to a viewer along an optimal line ofsight. The method includes: circularly polarizing light of a first imageso as to create circularly polarized light of a handedness; reflectingthe circularly polarized obliquely off of a broadbandreflector-polarizer so as to create reflected circularly polarized lightdirected to a converging reflector; reflecting the reflected circularlypolarized light from the converging reflector so as to: reverse thehandedness of the reflected circularly polarized light to createdreverse-handed reflected circularly polarized light; pass thereverse-handed reflected circularly polarized light through thebroadband reflector-polarizer; and focus the reverse-handed reflectedcircularly polarized light beyond the broadband reflector-polarizeralong the optimal line of sight so as to form a floating real image inspace of the entirety of the first image; and providing a second imagefor viewing in conjunction with the floating real image behind thefloating real image when viewed by a viewer along the optimal line ofsight.

In yet another embodiment, the present invention is directed to aprojection optic for projecting a real image of a source image into freespace, the source image provided via randomly polarized light. Theprojection optic includes: an optical axis extending through the sourceimage when the projection optic is in operative relation to the sourceimage; a circular polarizer located along the optical axis forpolarizing the randomly polarized light so as to created circularlypolarized light having a handedness; a path-folding optic located alongthe optical axis and configured to: receive the circularly polarizedlight from a first direction along the optical axis; pass a firstportion of the circularly polarized light as direct view light; reversethe handedness of a second portion of the circularly polarized light;and output reverse-handed circularly polarized light in a seconddirection opposite the first direction; wherein the path-folding opticincludes a beamsplitter and a broadband reflector-polarizer; a directview light extinguisher located along the optical axis for extinguishingthe direct view light and passing the reverse-handed circularlypolarized light in the second direction; and a converging element forfocusing the second portion of the circularly polarized light so as toform a floating real image in the second direction along the opticalaxis.

In a further embodiment, the present invention is directed to a displaysystem for displaying a plurality of overlaying images to a viewerpositioned along an optimal line of sight. The display system includes:an optical axis extending through the source image when the projectionoptic is in operative relation to the source image; a circular polarizerlocated along the optical axis for polarizing the randomly polarizedlight so as to created circularly polarized light having a handedness; apath-folding optic located along the optical axis and configured to:receive the circularly polarized light from a first direction along theoptical axis; pass a first portion of the circularly polarized light asdirect view light; reverse the handedness of a second portion of thecircularly polarized light; and output reverse-handed circularlypolarized light in a second direction opposite the first direction;wherein the path-folding optic includes a beamsplitter and a broadbandreflector-polarizer; a direct view light extinguisher located along theoptical axis for extinguishing the direct view light and passing thereverse-handed circularly polarized light in the second direction; and aconverging element for focusing the second portion of the circularlypolarized light so as to form a floating real image in the seconddirection along the optical axis.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, the drawings show a formof the invention that is presently preferred. However, it should beunderstood that the present invention is not limited to the precisearrangements and instrumentalities shown in the drawings, wherein:

FIG. 1 is a schematic diagram of an existing DBEF-based meniscus-typereal image optical system;

FIG. 2A is a schematic diagram of the meniscus-type real image opticalsystem of the present invention; FIG. 2B is a reduced-scale version ofthe schematic diagram of FIG. 2A illustrating how a viewer can view theentire floating real image from a vantage point different from thevantage point of FIG. 2A;

FIG. 3 is a schematic diagram of an alternative meniscus-type real imageoptical system of the present invention;

FIG. 4 is a schematic diagram of a cylindrical analog of the reflectiveelements of the optical systems of FIGS. 2A-B and 3 that may be used ineach of these systems;

FIG. 5 is a schematic diagram of a lens-type real image optical systemof the present invention; and

FIG. 6 is a schematic diagram of a real image optical system of thepresent invention having one or more secondary sources for forming oneor more secondary images in conjunction with primary image.

DETAILED DESCRIPTION OF THE INVENTION

A goal of the present invention is to project a brighter real image intospace while improving the off-angle response and manufacturability ofreal image optical systems, such as the optical system of U.S. Pat. No.6,262,841 discussed in the background section above in connection withFIG. 1 (optical system 20), using polarizing and reflecting techniquesto create a compact imaging system with a wide field of view. Severalimprovements for enhancing the image characteristics of such opticalsystems are disclosed. Generally, a basic premise of all threeperformance enhancements is either achieving greater “cooperation”between the polarizing elements (i.e. better, or “matched,” bandwidthresponse) or improving the losses due to absorption within thepolarizing elements, depending upon the enhancement. U.S. Pat. No.6,262,841 is incorporated by reference herein in its entirety and isreferred to below as the “Dike patent.”

Known optical system 20 of FIG. 1 utilizes a reflector-polarizer, suchas DBEF 40, that reflects one handedness of linear polarization whiletransmitting the other. The characteristic of differentiating betweenthe two states of polarization is critical when used in a polarizingsystem such as optical system 20, which relies on the existence of twostates such that one state is extinguished or absorbed while the otherstate is transmitted. Disadvantages of the DBEF-based design of opticalsystem are two-fold.

First, DBEF 40 works on linearly polarized light. The creation of twodistinct polarization states of light is possible by the characteristicof the handedness of circularly polarized light being switched viareflection. This reflection occurs at beamsplitter 48 of FIG. 1 andcauses the handedness of that portion of the light to be appropriate forreflection upon DBEF 40 and subsequent transmission through adjacentsecond linear polarizer 60 where it is then viewable as the desired realimage 52.

Similarly, source light 24 that might otherwise be viewable directlythrough optical system 20 without traveling along the “folded” opticalpath required for the formation of real image 52 is extinguished due toits polarization state being opposite of the polarization state of thedesired light. However, inefficiencies exist, since the desired lightmust be converted from the circularly polarized light required forreflection and polarization reversal at beamsplitter 48 to the linearlypolarized light required at DBEF 40. This conversion is accomplished byfirst quarter wave retarder 44 located between beamsplitter 48 and DBEF40. The desired light is required to pass through first quarter waveretarder 44 three times along the folded optical path to produce thepolarization state appropriate for passage through entire optical system20. Any deviation in the design value of first quarter wave retarder 44(usually 140 nm. i.e., one-quarter of 560 nm, the center wavelength ofthe spectrum of visible light) produced during manufacture or viaexposure to degrading ambient conditions is thus amplified by themultiple passes of the desired light through this retarder.

In order to avoid this performance degrading multiple pass-throughsituation at first quarter wave retarder 44, a design utilizing areflector-polarizer having broadband polarizing capability and theproperties of reflecting one handedness of circularly polarized lightwhile transmitting the other would be desirable because it could belocated immediately adjacent to concave beamsplitter 48. Such a designis illustrated in FIGS. 2A-B, which illustrates a meniscus-type opticalsystem 100 of the present invention. For the purpose of highlightingdifferences between optical system 100 of the present invention andknown meniscus-type optical system 20 of FIG. 1, optical system 100 ofFIGS. 2A-B is illustrated as having some elements in common with theoptical system of FIG. 1. However, as discussed below, other embodimentsutilizing various features of the present invention and different otherelements may certainly be made.

Referring to FIGS. 2A-B, a randomly-polarized source light 104 emittedand/or reflected from a source 108 is incident upon a circular polarizer112 comprising a first linear polarizer 116 and a first quarter waveretarder 120, or first quarter wave plate (QWP), to create circularlypolarized light. Then, the light passing through first quarter waveretarder 120 is made incident upon a broadband reflector-polarizer 124.First linear polarizer 116 and first quarter wave retarder 120 aremutually aligned so as to create a circular polarization of source light104 of an orientation appropriate for transmission through broadbandreflector-polarizer 124.

The circularly polarized light is partially reflected by a concavebeamsplitter 128 where its handedness of polarization is reversed bythis reflection. As the reflected portion now travels from right to leftin the figure, it is made incident upon broadband reflector-polarizer124, from which it is largely reflected since its handedness is oppositethe initially transmitted light. The light then travels left to right,passes partially through beamsplitter 128, is converted to linearlypolarized light by a second quarter wave retarder 132 and then passesthrough a second linear polarizer 136 to form desired real image 140.The combination of second quarter wave retarder 132 and second linearpolarizer 136 may be considered a direct-view light extinguisher 142,since it acts to substantially extinguish the portion of source light104, i.e., “direct view” light, transmitted through beamsplitter 128without having been also reflected by the beamsplitter. Optionally, thelight passing through second linear polarizer 136 may be directedthrough a wide view film 144 provided to enhance off-angle viewabilityof real image 140. In addition, the combination of broadbandreflector-polarizer 124 and beamsplitter 128 may be considered a“folded-path” optic because it reflects incoming light first in thedirection opposite from the direction of the incident light and then inthe direction of the incident light, thus folding the optical path.

As those skilled in the art will understand, in this example real image140 is floating in space when viewed from the vantage point 146illustrated in FIGS. 2A-B (which is also substantially the same as thesimilar vantage points in FIGS. 1, 3, 5 and 6). Although FIGS. 2A-B (andFIGS. 1 and 3-6 for that matter) are not drawn to any particular scale,they do indeed indicate exemplary relative sizes of the projectedfloating real image 140 and the projection optic (containing elements112, 124, 128, 142, 144, especially converging optic 128), as well asthe relative locations of the optic, floating image and vantage point146. For example, those skilled in the art will readily appreciate thatfor the viewers in FIGS. 2A-B to view the entirety of real image 140,the viewing “cone” 148, which is established by the extremities of thereal image as would be perceived by a viewer from vantage point 146,must intersect the optic at every point around the periphery of the coneat the optic. (For convenience the optic is taken as being at wide viewfilm 144 since FIGS. 2A-B represent elements largely functionally ratherthan physically, except as noted above; however, those skilled in theart will appreciate that the convergence actually begins at the initialreflection by converging optic 128.) In other words, for the viewer atvantage point 146 to view the entirety of real image 140, the entireimage must be “backlit” by the optic as viewed from vantage point 146.The closer real image 140 is to the optic the larger the viewing angleat which the entire image can be viewed. Also, the larger the optic, thelarger the viewing angle for viewing the entirety of real image 140.Conversely, the more nearly the image size approaches the optic size,the smaller the viewing angle becomes, until when the image and opticare of equal size, the viewing angle is zero, and the eye must be placedat infinity, with a pupil diameter equal to the image size, to see thewhole image.

Note that in FIGS. 2A-B, one viewer is show at vantage point 146,implying that viewing cone 148, projected backward through the foldedpath to converging optic 128, would intersect that optic at itseffective periphery. This further implies that the viewer, representedby the eye at vantage point 146 (which is the apex of viewing cone 148),may still view the entire floating image 140 while located anywherefurther from the image than point 146, as long as the eye remains withinthe boundaries of cone 148 as projected forward beyond point 146. Thisis illustrated in FIG. 2B with a viewing cone 150 that corresponds tothe vantage point 152 that is located further from converging optic 128that vantage point 146. The eye is therefore not restricted to a singlelocation for optimal image viewing as in many other prior artconfigurations, but may roam freely within the diverging portion ofviewing cone 148. Conversely, locating the eye closer to the image thanvantage point 146 would still allow part, but not all, of the image tobe seen from any given eye position; the eye would need to be moved fromplace to place in order to see different portions of the image.

Another implication is that the size of real floating image 140 mustalways be smaller than the converging optic 128, in order for image cone148 to converge at an eye-accessible vantage point 146. Some prior artavoids this requirement by creating a virtual image instead, therebyabandoning all the advantages of a real floating image which the presentinvention provides. Other prior art produces a real image but ignoresthis condition (e.g., slide or movie projectors that create images muchlarger than the converging optic), thereby abandoning the ability tolocate the eye at or beyond a vantage point 146 from which an entirefloating image may be observed directly, without the intervention of adiffuse viewing screen which causes the loss of 3D information. A simpleexample of this distinction may be observed by projecting an image on ascreen at relatively high magnification (i.e., with image size largerthan the projecting optic), then removing the screen and attempting toview the real image from beyond the screen. The result is that only avery small portion of the focused image (smaller than, or at most equalto, the projecting optic size) is visible from any chosen eye position,and that the eye must be moved to other positions to view differentsmall portions of the image. The larger the magnification of the image,the smaller the portion of the image that a viewer can view from anygiven viewing location.

Those skilled in the art will also appreciate that depending on thenature of image source 108, real image 140 may be a 2D image or a 3Dimage. For example, if source 108 is an illuminated object, say asphere, then real image 140 of the sphere will be perceived by a viewerfrom vantage point 146 (that is in this example assumed to be locatedwithin the stereopsis range of the viewer) to have 3D attributes,including depth. On the other hand, if source 108, for example, is avideo display displaying a 2D image, then real image 140 will likewisebe a 2D image regardless of whether or not the viewer is within theirstereopsis range. As another example, source 108 could be a 3D imageprojector that itself projects 3D images. For example, U.S. Pat. No.7,046,447 to Raber discloses a 3D image projector that can create 3Dimages from a series of 2D “image slices” using any one of a variety oftype of electrovariable optics (EVOs) that in rapid succession focus the2D image slices onto corresponding respective image planes to generate aperceived 3D image. In this case, real image 140 projected into spacewould be a 3D image of the 3D image created by the EVO projector. U.S.Pat. No. 7,046,447 is incorporated herein by reference to the extent ofits teachings of an image projector that could be used for source 108and sources in FIGS. 3, 5 and 6. Note that the 3D images therebyproduced may be directly viewed as floating in space, without theintervention of any viewing screen. Indeed, the interposition of such ascreen would require the selection of a particular plane within the 3Dimage to locate the screen, and the diffuse nature of such a screenwould result in an out-of-focus condition for all image information notfocused on the screen, and the consequent diffusion and loss of the 3Dinformation contained in the lost information which would otherwise havebeen focused on a plane other than the plane of the screen.

As in DBEF-based optical system 20 of FIG. 1, the initial portion ofpolarized source light 104 that transmits through beamsplitter 128without being reflected back towards source 108 is of a handednessopposite the handedness of the desired reflected light and is thereforeextinguished by direct-view light extinguisher 142. Important featuresof the present invention include: (1) utilizing broadbandreflector-polarizer 124 in lieu of DBEF, e.g., DBEF 40 of FIG. 1; (2)including wide view film 144 and (3) matching the frequency response offirst and/or second quarter wave retarders 120, 132 to the broadbandreflector-polarizer (or DBEF, if the broadband reflector-polarizer isnot utilized). Such quarter wave retarders may be achromatic.

Broadband reflector-polarizer 124 can be produced from either chiral orcholesteric liquid crystal (CLC) material. Generally, chiral liquidcrystal molecules are not superimposable on their mirror images.Cholesteric liquid crystal molecules are arranged in layers with theirlong axes parallel in each layer, and with a deliberate displacement insuccessive layers, thereby producing a helical stacking in thickness.These types of liquid crystal polarizers can exhibit propertiesdesirable for the contemplated imaging application due to a macroscopichelical structure, whose axes are perpendicular to the observationplane, that can be imparted into the liquid crystals of this type ofpolarizer. Polarizing elements of this type include cholestericstructure having a varying helical pitch distributed across thethickness of the element, thereby imparting a broadband responseappropriate for incorporation in a real-image optical system such asoptical system 100 of FIGS. 2A-B and optical systems described below,among others. This type of CLC structure is known as a “gradient pitch”CLC. Such gradient pitch CLCs are currently manufactured by Chelix ofSunnyvale, Calif. Rolic Technologies, Ltd. of Switzerland has a similarbroadband component that utilizes individually applied layers of liquidcrystal material each having a pitch characteristic intended to respondto a different wavelength range of light. Similarly, Philip Research ofEindhoven, the Netherlands, has developed material that uses a“deformed” helical structure to achieve broadband response.

In addition to the immediate advantage of working directly on thecircularly polarized light and eliminating quarter wave retarder 44(FIG. 1) that would otherwise be needed adjacent to DBEF 40, broadbandreflector-polarizer CLCs are largely non-absorptive, allowing forgreater overall efficiencies than DBEF. The new CLCs can have close to100% transmissibility for one handedness of circularly polarized lightand close to 90% reflectivity of the other. Thus, broadbandreflector-polarizer 124 offers a “stand alone” contribution to theoverall system throughput of about 90%. By contrast, DBEF has efficiencyof closer to 70% for each reflectivity and transmissibility, resultingin a contribution to the overall system throughput of about 49%.Additionally, current data supports the probability of achieving highercontrast, or more “cleanly” polarized light, than is currently possiblewith DBEF. A CLC polarizing material is a polymer and may be applieddirectly to a substrate (e.g., glass) to ensure good flatness andsubstantially no distortion in the final image originating fromlaminating to adjacent, non-rigid, polarizers. Consequently, such abroadband reflector-polarizer may be located at the outside of thelaminated series of polarizing elements and glass such that its flatnessis ensured without the need for additional substrates and anaccompanying manufacturing complexity.

Broadband reflector-polarizer 124 may also be located adjacent tobeamsplitter 128 such that no accompanying polarizers contribute topolarizing inefficiencies and poor final image quality. This isespecially significant at this location since any other transmissiveelement located there, such as first quarter wave retarder 44 inDBEF-based optical system 20 of FIG. 1, effectively appears three timeson the unfolded optical path.

It is noted that CLC polarizers have the characteristic of creating twohandednesses of circularly polarized light from randomly polarized,incident light. Thus, it is possible to exclude first linear polarizer116 and first quarter wave retarder 120 when broadbandreflector-polarizer 124 is a CLC polarizer so as to achieve a simplersystem with substantially fewer elements than shown in FIGS. 2A-B.However, the inclusion of first linear polarizer 116 and first quarterwave retarder 120 serve to more cleanly polarize source light 104 so asto increase contrast. Development of the other elements of opticalsystem 100 and realization of better cooperation of all of thepolarizers utilized, in terms of bandwidth response, may result in theexclusion of first linear polarizer 116 and first quarter wave retarder120.

The addition of wide-view film 144 can improve the off-angle viewabilityand contrast of optical system 100 relative to conventional real imageprojecting optical systems, such as optical system 20 of FIG. 1. Asdiscussed in the background section above, the birefringence of thepolarizing elements of optical system 20 leads to angularly-dependentlight transmission. Compensation films currently developed for LCDsoffer the ability to improve the quality of real image and are thereforesuitable for use as wide-view film 144. In addition, films such as thewide-view film (WVF) available from Fuji Photo Film, Inc., Greenwood SCand Rolic's Photo-Aligned LC-Polymer film are also suitable for use aswide-view film 144. It is recognized that there are many “wide-viewfilms” or compensating technologies that may achieve the desired effectin the contemplated applications. Implementation of these techniques mayrequire the addition of a component, 144, or different components atother, perhaps multiple, locations in the configuration. An importantaspect of the inclusion of wide-view film 144 is that its function andperformance enhancement contribution to the final image is independentof the system type, be it broadband reflector-polarizer-based orDBEF-based.

An additional area of improvement over the current generation ofDBEF-based real image display optical systems is the incorporation of“matched” first and second quarter wave retarders 120, 132 thatcooperate more advantageously with the bandwidth response of the otherpolarizing elements (specifically, either the broadbandreflector-polarizer of the present invention or the DBEF of aconventional DBEF-based optical systems. Inefficiencies that lead to adimmer image and greater bleed-through or contrast degradation resultwhen the different polarizing elements respond to different wavelengthswith different efficiencies. Obvious sources for this type of error arethe quarter wave retarders, e.g., first and second quarter waveretarders 44, 56 of optical system 20 of FIG. 1, the design wavelengthof which is simply centered in the visible spectrum, i.e., at 560 nm.Thus, such quarter wave retarders have decreasing efficiency atwavelengths higher and lower than 560 nm. These inefficiencies manifestthemselves primarily as increased bleed-through (or decreased contrast)in the final image, especially at oblique viewing angles.

Certain materials have superior performance and can be used in thequarter wave retarders, e.g., first and second quarter wave retarders44, 56 of FIG. 1 and first and second quarter wave retarders 120, 132 ofFIGS. 2A-B, to improve the overall image quality of the correspondingoptical system, e.g., optical systems 20, 100 respectively. Achromaticquarter wave retarders comprising liquid crystal polymers (LCPs) in astacked configuration that may be photo aligned, such as those producedby Rolic, have been found to exhibit superior performance, especiallywith respect to off-angle viewing characteristics. The bandwidthresponse of this quarter wave retarder may also be matched to thebandwidth characteristics of the DBEF or broadband reflector-polarizerand the type of light source (e.g., LCD) for improved efficiency and asuperior image.

FIG. 3 shows an alternative meniscus-type optical system 200 of thepresent invention. Optical system 200 is designed with recognition thatthe performance of polarizing elements, e.g., broadbandreflector-polarizer 124 of FIGS. 2A-B, is dependent upon the viewingangle. As described above, such dependence can result in contrast lossand color shift when real image 140 is viewed obliquely. Thus, it may beadvantageous to arrange the broadband reflector-polarizer in an opticalsystem in such a manner that the surface of the broadbandreflector-polarizer is mostly normal to a viewer as the viewing positionmoves to the left or right of the on-axis, or normal, position. Relativeto optical system 100 of FIGS. 2A-B, this goal may be accomplished byreversing the function of flat broadband reflector-polarizer 124 andconcave beamsplitter 128, such as shown in FIG. 3 wherein optical system200 contains a flat beamsplitter 204 and concave broadbandreflector-polarizer 208 that make up the folded-path optic. Thismodification results in identical optical mechanisms with respect to theuse of different polarization states within optical system 200 toachieve good ambient reflection attenuation and elimination of directviewing of source light.

In variations of meniscus-type optical systems of the present invention,such as optical systems 100, 200 of FIGS. 2A-B and 3, respectively, thereflective elements in each, i.e., concave beamsplitter 128 and flatbroadband reflector-polarizer 124 of FIGS. 2A-B and flat beamsplitter204 and concave broadband reflector-polarizer 208 of FIG. 3, may bereplaced by a pair of opposing cylindrical elements, e.g., cylindricalelements 300, 304 of FIG. 4, having their axes of curvature 308, 312orthogonal to one another. The properties and reasons for using opposingcylindrical elements 300, 304 in lieu of a corresponding sphericalelement are discussed in U.S. Pat. No. 4,653,875 to Hines, which isincorporated herein in its entirety. In such an arrangement, onecylindrical element, e.g., concave element 300, may be coated with aconventional beamsplitter coating and the other cylindrical element,e.g., cylindrical element 304, may comprise a broadbandreflector-polarizer element.

This design has two benefits. First, cylindrical broadbandreflector-polarizer 304 can be positioned such that its curvature axis312 is oriented for achieving the best viewing-angle performance.Second, there is a potential manufacturing improvement since the twocylindrical elements 300, 304 can be of a thin, flexible compositionsuch that the desired curvature can be obtained by bending thelaminations around a frame as described in the Hines patent. Thiseliminates the relatively expensive meniscus elements of FIGS. 2A-B and3, i.e., concave beamsplitter 128 (FIGS. 2A-B) and concave broadbandreflector-polarizer 208 (FIG. 3).

FIG. 5 illustrates another embodiment of an optical system 400 of thepresent invention. Optical system 400 differs from optical systems 100,200 of FIGS. 2A-B and 3 in that it utilizes a folded-path optic thatincludes a flat beamsplitter 404 and a flat broadbandreflector-polarizer 408, rather than having one or the other (or both inthe variation discussed in connection with FIG. 4) of these elementscurved. Consequently, the convergence power needed to display a realimage 412 is provided by one or more lenses, such as lens 416, that maybe located as shown in FIG. 5. Those skilled in the art will readilyappreciate that lens 416 may located otherwise, such as betweenbeamsplitter 404 and broadband reflector-polarizer 408. Also apparent isthe use of a lens as described above in combination with additionallenses or concave reflective elements to achieve image 412. Otherpossible variations of systems include variations similar to the opticalsystems shown in FIGS. 4-6 of the Dike patent.

Regardless of where lens 416 is located within optical system 400, thearrangement of the other elements, e.g., first and second linearpolarizers 420, 424 and first and second quarter wave retarders 428,432, and the polarization states of the light along the optical path maybe identical to the arrangement of the corresponding elements andpolarization states already described in connection with FIGS. 2A-B and3. The difference may only by the use of one or more lenses 416 in lieuof one or more concave elements to achieve the convergence that formsreal image 412. In an embodiment wherein lens 416 is located at eitherthe source or viewer side of optical system 400, a performanceimprovement over a simple source/lens system is achieved as either theimage or source side (in a real image creating optical system) of theoptical path is effectively folded three times by the reflection andre-reflection of the desired light between the reflective beamsplitter,e.g., beamsplitter 404, and broadband reflector-polarizer, e.g.,broadband reflector-polarizer 408. This can result in a devicecontaining a folded optical path being more compact than a devicecontaining a simple source/lens image system wherein the optical path isnot compressed (folded) in such manner. Similarly, if lens 416 islocated between beamsplitter 404 and broadband reflector-polarizer 408,a similar folded optical path advantage is created while the opticalpower of the lens is amplified by multiple passages through the lens.Such lens-based variations can result in decreased manufacturing costsas the reflective elements, i.e., beamsplitter 404 and broadbandreflector-polarizer 408, are flat, while the additional lens 416 can beof a conventional or Fresnel design.

FIG. 6 shows an embodiment of an optical system 500 of the presentinvention that can produce a floating real image 504 and one or morebackground images 508, 512 viewable behind the floating image.Generally, optical system 500 is arranged so as to eliminate a directview of source light 516 from a primary source 520 by locating thesource away from the viewing axis as well as by a crossed polarizermechanism. Randomly polarized source light 516 is circularly polarizedvia a circular polarizer 524, e.g., a linear polarizer/quarter waveretarder combination. This circularly polarized light is reflectedtoward a concave element 528 via an obliquely placed broadbandreflector-polarizer 532. Broadband reflector-polarizer 532 is orientedsuch that maximum reflection and minimum transmission of source light isachieved. The now leftward (relative to FIG. 6) traveling circularlypolarized light is then incident upon concave element 528, where thehandedness of circular polarization is reversed by reflection. Concaveelement 528 imparts the convergence that ultimately forms floating realimage 504. The rightward traveling light is now of a handednessappropriate for transmission through broadband reflector-polarizer 532,after which it then forms floating real image 504 viewable to a viewer536 when viewed along an optimal line of sight 538. When viewed alongoptimal line of sight 538, the viewer will typically see the entirety offloating real image 504 because of the “backlighting” provided by therelative large size of the optic, i.e., concave partially reflectingelement 528, compared with the floating real image. Those skilled in theart will readily appreciate that, depending on the size of the optic(concave element 528), floating real image 104 may be viewable in itsentirety at off-axis angles relative to optimal line of sight 538.

Real, or floating, images can be created conventionally by implementingreadily-available beamsplitting and concave reflecting elements in anarrangement similar to the arrangement shown in FIG. 6. However, suchelements are non-discriminating with respect to the polarization of thelight and as such have substantial losses at the obliquely placedbeamsplitter whose stand-alone contribution to the throughput of theentire system can be at best 25% (50% reflection followed by 50%transmission of that remaining portion of light). This scenario can beimproved upon by implementing a DBEF-based design as disclosed in theDike patent, e.g., in FIG. 9 thereof, wherein a quarter wave retarder(element 30 in FIG. 9 of the Dike patent) is required in front of theconcave element. Optical system 500 of FIG. 6, and particularly theimplementation of broadband reflector-polarizer 532 offers an additionalefficiency improvement while eliminating at least one laminatedpolarizing element relative to a DBEF-based design.

Conventional non-polarizing elements utilized in previous designs alsohave the disadvantage of allowing ambient light sources from the viewingenvironment to be reflected from the concave element, thereby creating acompeting light source image along with the desired image. Such ambientreflections result in a confusing image with low contrast. With respectto this ambient reflection detriment, the broadbandreflector-polarizer-based design of optical system 500 of the presentinvention first circularly polarizes incoming ambient light 540 and thenreverses the handedness of that polarization upon reflection fromconcave element 528. This “returning” light is then of a polarizationstate appropriate for reflection at broadband reflector-polarizer 532,which redirects the returning light away from viewer 536 such thatfloating image 504 is viewable without competing, contrast degrading,ambient light reflections. This mechanism is similar to a conventionalcircular polarizing anti-glare filter available for conventional videodisplays used in bright ambient conditions (aircraft cockpit displaysare a common application). These filters first circularly polarizeambient light, reverse the handedness at reflection upon the surface ofthe display itself and then absorb that light upon exiting the system. Adistinction relative to optical system 500 is that these filters absorbambient light as in a crossed-polarizer scenario, while the ambientlight controlling mechanism of the broadband reflector-polarizer-baseddesign of optical system 500 simply reflects the ambient light away fromviewer 536.

Additionally, conventional optical systems using non-polarizing elementsallow a viewer to see the image source directly by looking downward intothe devices containing the systems, wherein the image source is readilyviewable though the transmissive beamsplitter. In optical system 500 ofFIG. 6, however, it is possible to use the polarizing elements tolargely eliminate exiting light when one attempts to view primary source520 directly in the manner just described. In optical system 500,broadband reflector-polarizer 532 combines with the circular polarizer524 to function in a crossed-polarizer scenario to extinguish (or, moreaccurately, to redirect away from viewer 536) source light 516 whichmight otherwise exit optical system 500 so as to be viewable fromoutside the system.

An additional benefit of the broadband reflector-polarizer-based designof optical system 500 can be illustrated by comparison to conventional,non-polarizing, designs. Both the ambient reflection and the directsource light viewability detriments of non-polarizing designs can bepartially corrected by varying the geometry of the beamsplitter/concavereflector combination. Such a variation can be made so as to redirectambient reflections downward below the viewing axis by angling concaveelement 528 downward. This has the additional benefit of allowing thevertical source light axis to be rotated counterclockwise (when theconventional system is oriented with a viewer to the right in a mannersimilar to optical system 500 of FIG. 6), which moves the primary imagesource closer to the “front” of the display, thereby making the sourcemore difficult to view when looking downward into the display. Adrawback of this design, however, is that the off-axis nature of theconcave imaging element relative to the source due to varying thegeometry of the beamsplitter/concave reflector combination results in ageometrically confusing image with very poor vertical symmetry, amongother optical defects. These distortions are greatly amplified whenviewed from the right or left of the system. The broadbandreflector-polarizer-based design of optical system 500 allows a designerto maintain an on-axis arrangement of the imaging optic, i.e., concaveelement 528, and source illumination to creates a symmetrical andrelatively distortion-free image.

Concave element 528 can be either a fully-reflective element or apartially-reflective beamsplitting element. In the later case, thepartially transmissive nature of concave element 528 allows for theplacement of a background source 544 viewable in a direct manner throughconcave (beamsplitting) element 528 and broadband reflector-polarizer532. An additional background image 508 can be formed by the placementof an additional background source 548 above broadbandreflector-polarizer 532 such that an image reflected from thisadditional source is apparent behind floating real image 504. The resultof this variation is that either first or second background image 508,512 will be visible at the “rear” of optical system 500. Variation inthe placement of either background source 544 or background source 548can result in varying locations of background images 508, 512 relativeto floating real image 504. Concave element 528 can be of a focal lengthappropriate to create real image 504 that appears to float at somedistance in front of optical system 500, or it could be of a longerfocal length to create a virtual image that appears at infinity, as in aflight simulator, or at some intermediate finite distance.

While the present invention has been described in connection withcertain preferred embodiments, it will be understood that it is not solimited. On the contrary, it is intended to cover all alternatives,modifications and equivalents as may be included within the spirit andscope of the invention as defined above and in the claims appendedhereto.

1. A method of forming a real image floating in space, comprising:receiving light from an image source from a first direction along anoptical axis; polarizing the light so as to create circularly polarizedlight having a first handedness; bouncing a first portion of thecircularly polarized light between a broadband reflector-polarizer and apartially mirrored reflector along the optical axis so that the firstportion of the circularly polarized light has a second handednessopposite the first handedness and is directed along the optical axis ina second direction opposite the first direction; after said bouncing ofthe first portion of the circularly polarized light, converting thefirst portion of the circularly polarized light to linearly polarizedlight along the optical axis; passing a second portion of the circularlypolarized light through the broadband reflector-polarizer and thepartially mirrored reflector along the optical axis so that thecircularly polarized light retains the first handedness; after saidpassing of the second portion of the circularly polarized light,extinguishing the second portion of the circularly polarized light; andfocusing the first portion of the circularly polarized light to alocation in space so as to form a floating real image at the location.2. The method of claim 1, wherein said focusing of the first portion ofthe circularly polarized light includes focusing the first portion ofthe circularly polarized light along the optical axis in the seconddirection.
 3. The method of claim 1, wherein said focusing of the firstportion of the circularly polarized light includes focusing the firstportion of the circularly polarized light with the broadbandreflector-polarizer.
 4. The method of claim 1, wherein said focusing ofthe first portion of the circularly polarized light includes focusingthe first portion of the circularly polarized light with the partiallymirrored reflector.
 5. The method of claim 1, wherein said focusing ofthe first portion of the circularly polarized light includes focusingthe first portion of the circularly polarized light with a refractivelens.
 6. The method of claim 1, wherein said receiving of the light froman image source includes receiving the light from a 3D image and saidfocusing of the first portion of the circularly polarized light includesfocusing the first portion of the circularly polarized light so as toform a floating 3D real image.
 7. The method of claim 6, wherein saidreceiving of the light from the image source includes receiving thelight from a 3D image projector.
 8. The method of claim 7, wherein saidreceiving of the light from a 3D image projector includes receiving thelight from an electrovariable optic based projector.
 9. A method ofdisplaying a plurality of images to a viewer along an optimal line ofsight, comprising: circularly polarizing light of a first image so as tocreate circularly polarized light of a handedness; reflecting thecircularly polarized obliquely off of a broadband reflector-polarizer soas to create reflected circularly polarized light directed to aconverging reflector; reflecting the reflected circularly polarizedlight from the converging reflector so as to: reverse the handedness ofthe reflected circularly polarized light to created reverse-handedreflected circularly polarized light; pass the reverse-handed reflectedcircularly polarized light through the broadband reflector-polarizer;and focus the reverse-handed reflected circularly polarized light beyondthe broadband reflector-polarizer along the optimal line of sight so asto form a floating real image in space of the entirety of the firstimage; and providing a second image for viewing in conjunction with thefloating real image behind the floating real image when viewed by aviewer along the optimal line of sight.
 10. The method of claim 9,wherein the first image is a 3D image, the method further comprisingproviding the 3D image with a 3D image projector.
 11. The method ofclaim 10, wherein said providing of the 3D image includes providing the3D image with an electrovariable optic based 3D projector.
 12. Themethod of claim 9, wherein the concave reflector is a partially mirroredreflector and said providing of the second image includes providing thesecond image so that it is viewed from the optimal line of sight throughthe broadband polarizer-reflector and the partially mirrored reflector.13. The method of claim 9, wherein said providing of the second imageincludes providing the second image so that it is viewed from theoptimal line of sight as a reflection off of the broadbandreflector-polarizer.
 14. A projection optic for projecting a real imageof a source image into free space, the source image provided viarandomly polarized light, comprising: an optical axis extending throughthe source image when the projection optic is in operative relation tothe source image; a circular polarizer located along said optical axisfor polarizing the randomly polarized light so as to created circularlypolarized light having a handedness; a path-folding optic located alongsaid optical axis and configured to: receive the circularly polarizedlight from a first direction along said optical axis; pass a firstportion of the circularly polarized light as direct view light; reversethe handedness of a second portion of the circularly polarized light;and output reverse-handed circularly polarized light in a seconddirection opposite said first direction; wherein said path-folding opticincludes a beamsplitter and a broadband reflector-polarizer; a directview light extinguisher located along said optical axis forextinguishing the direct view light and passing said reverse-handedcircularly polarized light in the second direction; and a convergingelement for focusing the second portion of the circularly polarizedlight so as to form a floating real image in the second direction alongsaid optical axis.
 15. The projection optic of claim 14, wherein saidconverging element comprises said broadband reflector-polarizer.
 16. Theprojection optic of claim 14, wherein said converging element comprisessaid beamsplitter.
 17. The projection optic of claim 14, wherein saidconverging element is a refractive lens.
 18. A projector comprising theprojection optic of claim 14 and a 3D image projector for creating thesource image in 3D.
 19. The projector of claim 18, wherein said 3D imageprojector is an electrovariable optic based 3D image projector.
 20. Adisplay system for displaying a plurality of overlaying images to aviewer positioned along an optimal line of sight, comprising: a firstimage source for providing a source image via randomly polarized light;a first optical path extending from the source image; a circularpolarizer located along said first optical path for polarizing therandomly polarized light so as to created circularly polarized lighthaving a handedness; a converging element located along said firstoptical path for focusing a portion of the circularly polarized light toa point in space so as to create a floating real image, said convergingelement reversing the handedness of the circularly polarized light so asto create reversed-handed circularly polarized light; a broadbandreflector-polarizer positioned along said first optical path obliquelyrelative to each of said circular polarizer and said converging elementso as to reflect the portion of the circularly polarized light to saidconverging element, wherein said converging element is positionedrelative to said broadband reflector-polarizer so that thereverse-handed circularly polarized light is reflected through saidbroadband reflector-polarizer from said converging element in formingthe floating real image; and a second image source for providing abackground image to the floating real image when the viewer is locatedalong the optimal line of sight.
 21. The display system of claim 20,wherein said second image source is positioned to provide saidbackground image via reflection from said broadband reflector-polarizer.22. The display system of claim 20, wherein said second image source ispositioned to provide said background image via transmission througheach of said converging element and said broadband reflector-polarizer.23. The display system of claim 20, wherein said first image sourcecomprises a 3D image projector for creating the source image in 3D. 24.The display system of claim 23, wherein said 3D image projector is anelectrovariable optic based 3D image projector.